A study on the plasma-enhanced atomic layer deposition of amorphous inorganic oxides SiO and AlO on polypropylene (PP) was carried out with respect to growth taking place at the interface of the polymer substrate and the thin film employing in situ quartz-crystal microbalance (QCM) experiments. A model layer of spin-coated PP (scPP) was deposited on QCM crystals prior to depositions to allow a transfer of findings from QCM studies to industrially applied PP foil. The influence of precursor choice (trimethylaluminum (TMA) vs [3-(dimethylamino)propyl]-dimethyl aluminum (DMAD)) and of plasma pretreatment on the monitored QCM response was investigated. Furthermore, dyads of SiO/AlO, using different Al precursors for the AlO thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO and AlO from TMA on scPP is significantly hindered if no oxygen plasma pretreatment is applied to the scPP prior to depositions, the DMAD process was found to yield comparable AlO growth directly on scPP similar to that found on a bare QCM crystal. From this, the interface formed between the AlO and the PP substrate is suggested to be different for the two precursors TMA and DMAD due to different growth modes. Furthermore, the residual stress of the thin films influences the barrier properties of SiO/AlO dyads. Dyads composed of 5 nm AlO (DMAD) + 5 nm SiO exhibit an oxygen transmission rate (OTR) of 57.4 cm m day, which correlates with a barrier improvement factor of 24 against 5 when AlO from TMA is applied.

The adhesion of thin CVD films on polyolefins is often critical due to the low surface free energy of the polymers. In this study, injection moulded PP samples are produced and investigated. The samples are treated in very well-characterized pulsed plasmas before a HMDSO-based coating is applied. The resulting bond strength is analyzed using pull-off tests. The fractured interfaces are characterized with XPS. Oxygen and argon plasma pre-treatments of the PP samples result in a bond strength improvement by a factor of about 2. Comparing oxygen and argon pre-treatments at equal ion fluences to the surface, it can be shown that the bond strength between CVD-coating and polymer is similar.

The properties of plasma-enhanced chemical vapour deposition (PECVD) coatings on polymer materials depend to some extent on the surface and material properties of the substrate. Here, isotactic polypropylene (PP) substrates are coated with silicon oxide (SiOx) films. Plasmas for the deposition of SiOx are energetic and oxidative due to the high amount of oxygen in the gas mixture. Residual stress measurements using single Si cantilever stress sensors showed that these coatings contain high compressive stress. To investigate the influence of the plasma and the coatings, residual stress, silicon organic (SiOCH) coatings with different thicknesses between the PP and the SiOx coating are used as a means to protect the substrate from the oxidative SiOx coating process. Pull-off tests are performed to analyse differences in the adhesion of these coating systems. It could be shown that the adhesion of the PECVD coatings on PP depends on the coatings’ residual stress. In a PP/SiOCH/SiOx-multilayer system the residual stress can be significantly reduced by increasing the thickness of the SiOCH coating, resulting in enhanced adhesion.

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